Nanocrystalline semiconductors and oxides have attracted a great deal of research interest in recent years due to their fundamental importance in bridging the gap between bulk matter and molecular species. See A. P. Alivisatos, “Perspectives on the Physical Chemistry of Semiconductor Nanocrystals,” J. Phys. Chem., 100, 13226-13239, (1996); A. S. Edelstein and R. C. Cammarata, Nanomaterials: Synthesis, Properties and Applications, Eds. Institute of Physics Publishing: Bristol, 1996. As a result of quantum confinement effects, nanosized metal oxide particles can possess electronic bandgaps that are larger than the corresponding bulk material. Furthermore, the bandgap of these quantum-confined nanosized metal particles can be tuned within the nanosized regime by changing and/or mixing the particle sizes. See L. Li et al., “Band Gap Variation of Size- and Shape-Controlled Colloidal CdSe Quantum Rods,” Nano Letters, 1, 349-351 (2001). The tunable optoelectronic properties of such nanosized metal oxide particles will likely provide them with performance characteristics that far surpass conventional metal oxide particles possessing sizes on the order of micrometers.
Current research efforts in the area of nanocrystalline semiconductors and oxides are driven, to a great extent, by the many applications in which such nanosized particles are expected to find use. Such applications include novel optical, electrical, and mechanical devices, photovoltaic solar cells, light-emitting diodes, varistors, light catalysts, gas sensors, optoelectronic devices, optical switches, UV absorbers, nano-lasers, ion-insertion batteries, electrochromic devices, etc. See A. S. Edelstein and R. C. Cammarata, Nanomaterials: Synthesis, Properties and Applications, Eds. Institute of Physics Publishing: Bristol, 1996; B. O'Regan and M. Grätzel, “A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films,” Nature, 353, 737-740 (1991); M. A. Fox and M. T. Dulay, “Heterogeneous Photocatalysis,” Chem. Rev., 93, 341-357 (1993); V. L. Colvin, M. C. Schlamp and A. P. Alivisatos, “Light-Emitting Diodes Made from Cadmium Selenide Nanocrystals and a Semiconducting Polymer,” Nature 370, 354-357 (1994); J. Lee, J.-H. Hwang, J. J. Mashek, T. O. Mason, A. E. Miller and R. W. Siegel, “Impedance Spectroscopy of Grain Boundaries in Nanophase ZnO,” J. Mater. Res., 10, 2295-2300 (1995); M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo and P. Yang, “Room-Temperature Ultraviolet Nanowire Nanolasers,” Science, 292, 1897-1899 (2001). Indeed, such nano-sized particles have already found commercial application in sunscreens and cosmetics as the active UV absorbing ingredient (U.S. Pat. No. 6,171,580 to Katsuyama et al.).
The breadth of potential applications available for nanosized metal oxide particles has fueled additional research into their synthesis and manufacture. Nanosized metal oxide particles can presently be produced by a variety of methods, including chemical gas phase growth methods such as chemical vapor deposition (CVD) and metal-organic chemical vapor deposition (MO-CVD), molecular beam epitaxial methods, and plasma synthesis methods. See U.S. Pat. No. 5,128,081 to Siegel et al. and U.S. Pat. No. 6,580,051 to Peterson et al. These methods, however, require expensive and complex equipment. On the other hand, sol-gel methods can produce nanosized metal oxide particles at relatively low temperatures with relatively inexpensive equipment, as compared to the above-mentioned methods. As a result, these sol-gel methods have gained popularity in recent years.
Zinc oxide (ZnO) is a particularly interesting metal oxide material from which nano-sized metal oxide particles can be made. ZnO, which is one of the few wide-bandgap semiconducting oxides that shows quantum confinement effects in an experimentally accessible size range, is a simple, stable species that, when obtained in clusters known as nanocrystals (quantum dots) with diameters below about 7 nm, has unique physical, chemical and optical properties not observed in the corresponding bulk solid. These quantum size effects, combined with the large surface area-to-mass of the dots, make ZnO nanoparticles highly desirable when seeking to exploit such quantum confinement in applications ranging from optics to high strength materials to data storage. Moreover, compared with other wide-bandgap semiconductors, ZnO has a very high exciton binding energy of 60 meV as a light emitter; GaN, by comparison, is 25 meV. For electronic applications, its attractiveness lies in having a high breakdown strength and high saturation velocity. ZnO is also much more resistant to radiation damage than other common semiconductor materials, such as Si, GaAs, CdS, and even GaN. See D. C. Look, “Recent Advances in ZnO Materials and Devices,” Materials Science and Engineering, B80, 383-387 (2001).
As with the preparation of other nanosized metal oxide particles, there are many methods developed to prepare nanosized ZnO, such as chemical vapor deposition, molecular beam epitaxy, metal-organic vapor-phase epitaxy and colloid preparation. The colloidal (sol-gel) preparation is widely used among these approaches because it is easy to perform. The seminal paper describing the synthesis of ZnO nanoparticles using a sol-gel method is: Bahnemann et al., “Preparation and Characterization of Quantum Size Zinc Oxide: A Detailed Spectroscopic Study,” J. Phys. Chem., 91, 3789-3798 (1987). This paper describes a method whereby ZnO nanoparticles are formed by the addition of a metal hydroxide species to a zinc acetate-alcohol solution. Subsequent to such disclosure, highly concentrated ZnO particles, with particle sizes in the range of 2-7nm, were obtained using a sol-gel method involving the addition of metal hydroxide to a zinc acetate-alcohol solution, as described in Meulenkamp, “Synthesis and Growth of ZnO Nanoparticles,” J. Phys. Chem. B, 102, 5566-5572 (1998). This work further describes a method for monitoring particle size by optical absorption and luminescence spectroscopy, whereby ZnO particle size was found to be dependent on reaction time and temperature. This method, however, additionally leads to the formation of large particles when carried out under high concentrations of the metal oxide precursor (e.g., ˜0.1M).
Wong et al. have studied the kinetics involved in ZnO nanoparticle growth [E. M. Wong, J. E. Bonevich and P. C. Searson, “Growth Kinetics of Nanocrystalline Particles from Colloidal Suspensions,” J. Phys. Chem. B, 102, 7770-7775 (1998)], as well as the effect of organic capping ligands on such ZnO nanoparticle growth [E. M. Wong, P. G. Hoertz, C. J. Liang, B. M. Shi, G. J. Meyer and P. C. Searson, “Influence of Organic Capping Ligands on the Growth Kinetics of ZnO Nanoparticles,” Langmuir, 17, 8362-8367 (2001)]. However, this method is time consuming and gives very low yields in terms of the relative concentration of ZnO in the resulting sol. Highly concentrated ZnO colloids have been prepared using a sol-gel synthesis developed by L. Spanhel and M. A. Anderson in 1991 [L. Spanhel and M. A. Anderson, “Semiconductor Clusters in the Sol-Gel Process: Quantized Aggregation, Gelation, and Crystal Growth in Concentrated ZnO Colloids,” J. Am. Chem. Soc., 113, 2826-2833 (1991)]. According to that procedure, a Zn2+ precursor was prepared by refluxing a Zn(OAc)2.2H2O-ethanol solution, followed by the addition of LiOH.H2O powder into the solution. E. A. Meulenkamp modified this method by using a LiOH.H2O-ethanol solution and lowering the reaction temperature to 0° C. See E. A. Meulenkamp, J. Phys. Chem. B, 102, 5566-5572 (1998).
Based on current understanding, the growth of ZnO nanocrystal clearly depends on reaction time, temperature and the stoichiometry of the reactants. However, the detailed reaction mechanism is not yet clear. It has been claimed that the use of other bases, such as KOH, NaOH, Mg(OH)2, and other alcohols, such as methanol and propanol, failed to achieve desirable results. See L. Spanhel and M. A. Anderson, J. Am. Chem. Soc., 113, 2826-2833 (1991). In addition, low temperature processing is not favored for some in situ organic material-based nanocomposite preparations. Consequently, there is a demonstrated need to address the above-mentioned problems and uncertainties.
As a result of the above-described state of the art, it would be highly desirable to find a novel sol-gel method that can produce nanosized metal oxide particles at high concentrations without the co-formation of large (non-nanosized) particles. Additionally, any such method(s) capable of producing nanosized ZnO would be especially desirable.